Group III nitride semiconductors are compound semiconductor materials of an aluminum (Al) atom, a gallium (Ga) atom, and an indium (In) atom which are group IIIB elements (hereinafter referred to simply as III elements) and a nitrogen (N) atom which is a group VB element (hereinafter referred to simply as a group V element). That is, compound semiconductor materials obtained as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) and their mixed crystals (AlGaN, InGaN, InAlN, InGaAlN) are group III nitride semiconductors.
Elements using the group III nitride semiconductors include optical devices such as light-emitting diodes covering a wide wavelength region over far-ultraviolet/visible/near-infrared regions (LED: Light Emitting Diode), laser diode (LD), solar cell (PVSC: Photovoltaic Solar Cell), photodiode (PD) and the like and electronic devices such as high electron mobility transistor for high-frequency/high-output applications (HEMT: High Electron Mobility Transistor), metal-oxide-semiconductor field effect transistor (MOSFET) and the like.
In order to realize element applications as above, it is necessary to epitaxially grow a group III nitride semiconductor thin film on a single crystal substrate so as to obtain a high-quality single crystal film with few crystal defects (epitaxial film). However, since a single crystal substrate made of a group III nitride semiconductor is extremely expensive, it is not used except some applications, and a single crystal film is obtained mainly by hetero-epitaxial growth of sapphire (α-Al2O3), silicon carbide (SiC) on dissimilar substrates.
For the epitaxial growth of such group III nitride semiconductor thin film, a metal organic chemical vapor deposition (MOCVD) method by which high-productivity and high-quality epitaxial films can be obtained is used. However, the MOCVD method has problems that a production cost is high, particles can be easily generated and it is difficult to obtain high yield and the like.
On the other hand, a sputtering method has features that a production cost can be suppressed low and particle generation probability is also low. Therefore, if at least a part of a film forming process of the group III nitride semiconductor thin film can be replaced by the sputtering method, it is likely that at least a part of the above-described problems can be solved.
However, the group III nitride semiconductor thin film fabricated by using the sputtering method has a problem that a crystal quality tends to become poorer than those fabricated by using the MOCVD method. For example, crystalline of the group III nitride semiconductor thin film fabricated by using the sputtering method is disclosed in NPL 1, for example. In NPL 1, a GaN film with c-axis orientation is epitaxially grown on an α-Al2O3 (0001) substrate by using a high-frequency magnetron sputtering method. NPL 1 describes that in X-ray rocking curve (XRC) measurement of a GaN (0002) plane, its full width at half maximum (FWHM) is 35.1 arcmin (2106 arcsec). This value is an extremely large value as compared with a GaN film on the α-Al2O3 substrate currently sold in the market and indicates that mosaic expansion of tilt which will be described later is large and crystal quality is poor.
Here, concepts used as indexes indicating the crystal quality, that is, (1) mosaic expansion of tilt, (2) mosaic expansion of twist, and (3) polarity will be described in brief. The mosaic expansion of tilt in (1) indicates a degree of variation in crystal orientation in a substrate perpendicular direction, and the mosaic expansion of twist in (2) indicates a degree of variation in crystal orientation in a substrate in-plane direction. The polarity in (3) is a term meaning an orientation of a crystal, and in the case of c-axis orientation film, there are two types of growth modes, that is, +c polarity and −c polarity. The growth, with the +c polarity corresponds to (0001) orientation and the growth with −c polarity corresponds to (000-1) orientation.
It is necessary for a single crystal with favorable crystalline that mosaic expansion of tilt and twist is small and also, and that polarity is uniformly either of +c polarity or −c polarity. Since a group III nitride semiconductor thin film with favorable morphology and excellent crystalline can be easily obtained with the +c polarity, in particular, establishment of a process of obtaining group III nitride semiconductors with the +c polarity is in demand. On the other hand, many attempts have been made in order to obtain a good-quality group III nitride semiconductor thin film by the sputtering method (See PTLs 1 and 2).
PTL 1 discloses a method of realizing higher quality of group III nitride semiconductor thin films by applying plasma processing to a substrate before a group III nitride semiconductor thin film (AlN in PTL 1) is formed on an α-Al2O3 substrate by using the sputtering method or particularly a method of obtaining a group III nitride semiconductor thin film with extremely small mosaic expansion of tilt.
Moreover, PTL 2 discloses a manufacturing method of a group III nitride semiconductor (group III nitride compound semiconductor in PTL 2) light-emitting element in which a buffer layer (an intermediate layer in PTL 2) made of the group III nitride semiconductor (group III nitride compound in PTL 2) is formed on a substrate by the sputtering method and an n-type semiconductor layer provided with a base film, a light-emitting layer, and a p-type semiconductor layer are sequentially laminated on the buffer layer made of this group III nitride semiconductor.
PTL 2 describes that, as a procedure for forming the buffer layer made of the group III nitride semiconductor, a pre-treatment process of applying plasma processing to a substrate and a process of depositing the buffer layer made of the group III nitride semiconductor by sputtering method subsequently to the pre-treatment process. Moreover, in PTL 2, as a preferable mode of the substrate and the buffer layer made of the group III nitride semiconductor, an α-Al2O3 substrate and AlN are used, and as a deposition method of the n-type semiconductor layer provided with a base film, the light-emitting layer, and the p-type semiconductor layer, the MOCVD method is preferably used.